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198 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
TABLE 11-3. Maximum Acceptable Vibrations
Pump speed (rpm) Displacement (Mils, peak-peak)
3600 1.25
1800 2.50
1200 3.75
Note: Displacement is measured perpendicular to the shaft axis at a point
on the bearing housing adjacent to each bearing.
Table 11-3 lists generally accepted vibration 6. Use a suction strainer to keep rags, rubber
limits. If vibration exceeds these limits, corrective chunks, sticks, wrenches, and so forth, out of
action should be taken. the pump impeller. Clean the strainer before
it clogs.
7. Size the centrifugal-pump piping by flow rate
requirements of the equipment, not by the size
SUMMARY of a flange on a piece of equipment or on the
centrifugal pump.
8. Never adjust a valve in the suction line to
1. Centrifugal pumps produce a constant head. regulate flow rate or pressure of the pump.
When the mud weight changes, the pressure This will cause the pump to cavitate and de-
changes but the head remains constant. stroy itself. Cavitation sounds as if gravel is
2. Head may be converted into pressure using passing through the centrifugal pump.
the well control equation: 9. The discharge valve in the piping next to the
centrifugal pump may be closed briefly to mea-
P = 0.052 (MW, ppg) (depth, ft) sure the discharge pressure. This is a diagnos-
tic tool. The no-flow centrifugal pump discharge
Depth and head are the same. A centrifugal pressure can be used to monitor the wear on
pump will produce a constant head but the an impeller without dismantling the pump.
pressure will depend on the mud weight. 10. Never walk away from a discharge valve that
3. Think of a centrifugal pump as a constantly you have closed. The fluid in the pump will
filled water tower containing drilling fluid. The boil after running for only a few minutes. Cen-
flow rate from the tower depends on the pip- trifugal pumps use less horsepower when the
ing attached to it. For example, the flow rate discharge valve is closed. The current to an
would be different for 20 feet of 6-inch pipe electric motor will be lower so there is no
compared to 2,000 feet of -J-inch pipe. The danger of blowing a circuit breaker by clos-
same head would be applied to both piping ing the discharge valve.
systems. The pressure at the bottom of the 11. When pumping into a long, empty line with a
tower would depend on the mud weight. centrifugal pump, partially close the discharge
4. Centrifugal pumps will produce the same head valve until the line is full. A centrifugal-pump
with 8.34 ppg water as with 18 ppg of drilling motor requires less current (or horsepower)
fluid. The horsepower requirement to pump when nothing is connected to the discharge
the heavier weight mud is higher by a ratio of side of the pump than when the discharge side
mud weights. is completely closed. Always fully open the
5. Never operate two centrifugal pumps in valve before leaving it.
parallel supplying the same piece of equip- 12. Do not open bypass valves in the piping sys-
ment. In other words, two pumps should tem to reduce flow rate or pressure on equip-
not pump through the same pipe at the same ment being supplied by a centrifugal pump.
time. Operating two centrifugal pumps in par- This wastes power (or energy) and may over-
allel is equivalent to connecting two water load the motor.
towers together. Usually, when this occurs in 13. A flooded suction is needed to prime a centri-
a drilling fluid system, one pump will have fugal pump and prevent cavitation. When a
fluid flowing in a reverse direction because the flooded suction is not possible, make certain
heads are not identical. that the proper NPSH is available.
